The present invention relates to a method of making a polymer-based composite material having improved mechanical and electrical properties, and more particularly, to a polymer composite which is reinforced with a blend of carbon nanofibers and nano-scale particles.
Many applications exist where there is a need for the enhancement of conductivity in polymer materials: for example, in static electricity dissipation, automotive applications such as electrostatic painting of panels, and electromagnetic interference (EMI) shielding applications. Typically, polymers may be reinforced with additives including talc, carbon black, milled carbon fibers, or metallic particles to improve modulus of elasticity, or electrical conductivity, or a combination of these and other physical properties. The incorporation of such additives typically has a dramatic influence on the viscosity of the polymer, and, as a consequence, on the processing characteristics of the compound when it is formed into a composite article through extrusion and molding.
The use of nanocarbon materials has been proposed to provide mechanical reinforcement to a polymeric matrix. For example, polymer composites containing carbon nanofibers dispersed in a polymeric matrix have been known to exhibit mechanical properties such as stiffness, strength and toughness, and physical properties such as coefficient of thermal expansion, electrical and thermal conductivities which are superior to those of the polymeric matrix alone. Moreover, they do so at much lower loading levels than are required to reach comparable physical property thresholds using milled carbon fibers or carbon black. As a result, the processing characteristics of the polymer compound are modified to a much lower degree than polymer compounds synthesized by incorporation of milled carbon fiber or carbon black.
Uniform dispersion of the nanomaterial into the host polymer helps to obtain the full benefit of the nanomaterial in modifying the properties of the host polymer. It has been shown that blending carbon nanofibers with larger diameter milled glass fibers has a beneficial effect in achieving a satisfactory dispersion of the nanofibers as a consequence of bi-modal mixing of two disparate geometries of material (U.S. Pat. No. 6,911,169, which is incorporated herein by reference). Such blending of two or more particle sizes may reduce the shear force expressed on nanofibers during compounding, so that the nanofibers retain a high aspect ratio and more efficiently contribute to a connected network of conductors needed to achieve electric pathways within the polymer.
One type of carbon nanofiber which has been used in polymeric composites is vapor grown carbon fibers. A method for producing vapor grown carbon nanofibers is taught in U.S. Pat. No. 5,024,818, the disclosure of which is hereby incorporated by reference. These fibers are significantly smaller than carbon fibers produced by other conventional methods. The diameter of these carbon nanofibers is typically from about 80 to 200 nm, In addition, the fibers are relatively short, with lengths ranging from about 40 to about 200 micrometers.
However, there is a significant difficulty when vapor grown carbon fibers are incorporated in a polymer matrix. A high shear bulk fabrication technique is typically used, such as, for example, a twin screw extruder. The high shearing forces of the equipment, which are designed to promote dispersion of the fibers, have a counter effect on the fiber length and network formation, destroying the interconnections between the fibers that are necessary for thermal and electrical conductivity in the composite. Such an undesirable effect is particularly evident with the use of small diameter vapor grown carbon fibers, whose high surface area and stiffness can render the fibers too fragile for many types of production mixers. The result is a composite material whose electrical resistivity is significantly higher than that desired. While higher carbon fiber contents of 5 volume percent or higher can help reduce resistivity, such composites are more difficult to process and can exhibit unacceptable mechanical properties.
Accordingly, there is a need in the art for a method of providing carbon-filled polymer-based composites having improved electrical properties and suitable mechanical properties for electrical applications.